Research Abstract Depression is the single largest health care burden in the world; no other illness accounts for even half its burden. Recently, the World Bank, WHO, and NIMH highlighted the crippling economic costs of unaddressed depression and the need to scale up depression care globally. In Vietnam, we have found collaborative care to be effective and have conducted clustered randomized control trial that has shown large effect sizes for a Multicomponent Collaborative Care Model for Depression (MCCD), in which depression care was task-shifted to primary care providers in local community health clinics and supported by mobile psychiatrists. While the benefits of collaborative care for depression are well-established, the most appropriate and effective implementation strategies for scaling up this model have not been identified. To address this knowledge gap, our R01 proposal will build on our team's previous work on the MCCD and leverage current policy initiatives related to depression care in Vietnam to identify implementation models most likely to lead to successful implementation and sustainment of effective services in low resource settings. We will partner with local and national community and government organizations to conduct a randomized controlled trial (RCT) comparing effectiveness of three implementation models: (a) usual implementation (UI), which typically includes one workshop and toolkit; (b) enhanced supervision (ES; includes model UI); and (c) community-engaged learning collaborative (CELC; includes ES). According to the RE-AIM Implementation Evaluation framework, to have a population-level impact, an EBI must be adopted by providers, reach a large proportion of the targeted patient population, be implemented with fidelity, effectively improve outcomes, and be maintained after research funds are withdrawn, thus we will assess implementation outcomes at the organizational, provider, and patient levels based on this framework. Additionally, we will assess organizational and provider factors associated with successful implementation and measure incremental cost-effectiveness of each implementation strategy. Comparing implementation models on RE-AIM implementation outcomes and cost-effectiveness can guide policy decisions on best strategies to support scale-up of collaborative care models for depression and other EBIs. This comprehensive and rigorous implementation effectiveness study will contribute to our understanding of the added value of using a CELC strategy over and above the best practice in training (ES), which has not been done even in high-income countries. This timely evaluation of an depression care task- shifting will provide much needed knowledge about what implementation strategies and factors promote adoption, delivery, and sustainment of high-quality depression care in low-resource settings where limited mental health human resources are available, addressing global priority knowledge gaps in implementation science and mental health research.